1. Field of Invention
This disclosure is in the field of Synthetic Aperture Radar (SAR) and imaging obtained from a stationary or moving platform where platform motion is not required for SAR imaging.
2. Description of the Related Art
Synthetic Aperture Radar (SAR) is used for ground mapping as well as target identification. A general principle behind SAR is to coherently combine the amplitude and phase information of radar returns from a plurality of sequentially transmitted pulses. These pulses are from a relatively small antenna on a forward moving platform. As the platform moves, the information contained in the radar returns acquired at different positions is coherently combined to arrive at a SAR image. An increase in resolution of SAR, as compared to a stationary antenna of the same size, comes from the forward motion of the SAR antenna and collection of multiple returns along the path covered by the SAR antenna. The SAR image is typically side looking. That is, the path of the moving platform is not toward the location of the SAR image, but rather tangential to it.
As is well known in the art, the plurality of returns creating a SAR image generated by the transmitted pulses along a forward path of the platform make up an array. During the array, amplitude as well as phase information returned from each of the radar returns, for each of many range bins, is preserved. That is, the SAR image is formed from the coherent combination of the amplitude and phase of radar return(s) within each range bin, motion compensated for forward spatial displacement of the moving platform during the acquisition of the returns for the duration of the array.
Motion compensation for platform movement is a necessity of SAR. Motion compensation for spatial displacement, or forward motion of the platform, shifts the phase of each radar return (typically an I+jQ complex quantity derived from an analog to digital converter) in accordance with the motion of the moving antenna with respect to a reference point. The SAR imaging process depends on the coherent, phase accurate summing of all radar returns expected within an array. These principles are detailed in the prior art as exemplified by W. G. Carrara, R. S. Goodman and R. M. Majewski in Spotlight Synthetic Radar, Boston, Artech House, 1995, incorporated herein in its entirety by reference.
In one solution to motion compensation, as an approximation, the plurality of returns creating a SAR image from different platform positions are assumed to be acquired along a presumed known, planar and rectilinear path of the platform to make up an array. If the SAR platform follows the proper path, during the array, amplitude as well as phase information returned from each of the pulses, for each of many range bins, is preserved. Motion compensation of target returns is critical to SAR imaging. The clarity of details of a SAR image is dependent on the accuracy of the motion compensation applied to each radar return prior to SAR image computation as well as relative position of the platform with respect to the ideal path required for SAR image acquisition. The SAR imaging process depends on the coherent, phase accurate summing of the sequence of all radar returns expected within an array, typically accurate to better than ¼ wavelength. Accelerometers coupled to an inertial navigation system (INS) are typical prior art approaches to determining the position in space of the transmit/receive radar antenna phase center.
The essential aspect of motion compensation to SAR and its inherent inaccuracies and complexities have brought imaginative solutions. Where a non-ideal platform motion exists, having forward flight path deviations away from a rectilinear, planar path, motion compensation (INS based) of the pulses may not fully compensate for phase errors. Consequently, the coherent summing of radar returns over an array will also be inaccurate. The resulting phase error over the array blurs the SAR image, reducing its utility. Where INS based motion compensation is insufficient, one solution is autofocus. Autofocus is applied where the accuracy of the INS derived motion compensation phase compensation applied to each radar A/D sample is insufficient. Autofocus (AF) methods typically use information contained in the radar returns of the SAR data itself in an attempt to phase align radar return samples to accuracies better than those available from INS motion compensation alone. Estimated phase error derived from collected SAR data is applied to the motion compensated SAR data to improve the resulting SAR image.
Another problem in the prior art is that SAR is side looking. That is, the SAR platform is required to have a component of motion tangential to the target to generate a SAR image. Without platform motion, the principles of SAR cannot be applied. Thus, in the prior art, the SAR platform is precluded from loitering in one, hovering position. For example, a helicopter could not vertically peek over a hill to acquire a SAR image, as it would have to move forward, as explained above. In the prior art, during the acquisition of a SAR image, the helicopter is denied the safety of a hill to shield it from enemy fire because of the required tangential motion.
Tangential motion presents yet another problem of prior art SAR. SAR acquires an image towards the side, at an angle, to the forward motion vector of the platform. This typically precludes using SAR for imaging a landing site or target where the landing site or target is directly ahead, aligned with the longitudinal axis of the platform. Thus, in the prior art, SAR radar cannot be efficiently used for radar imaged terrain following, or landing site imaging as the SAR image presented is not in front of the path of the platform, but rather to the side.
As detailed above, prior art SAR has imposed two major constraints that limit its utility. The first is the need for transmit/receive motion of the SAR platform to complete a SAR image and complexities of motion compensation. The second is the side looking limitation, precluding mapping directly ahead of the flight path.